1. Field of the Invention
The present invention relates to an electrostatic protection device for a semiconductor device, and more particularly to an electrostatic protection device for a semiconductor device, which protects a semiconductor integrated circuit from damage due to electrostatic discharge (ESD).
2. Description of the Prior Art
As generally known in the art, electrostatic discharge (ESD) is one of main factors affecting the reliability of a semiconductor chip. This electrostatic discharge causes damage to a semiconductor chip when the semiconductor chip is handled, or installed in a system. Accordingly, an electrostatic protection device is indispensably provided to a data input/output area of a semiconductor device in order to protect the semiconductor device from static electricity. If an electrically charged human body or machine makes contact with a semiconductor device, static electricity charged in the human body or the machine is discharged inside of the semiconductor device through an input/output port of an external pin in the semiconductor device, so that excessive electrostatic current having high energy may inflict fatal damage on an internal circuit of the semiconductor device. Most semiconductor devices include an electrostatic protection device between an input/output port and an internal circuit of the semiconductor device in order to prevent an internal main circuit from being damaged by the static electricity.
In the meantime, as a technique of manufacturing a semiconductor device becomes developed, the thickness of a gate insulating layer of a transistor included in an input/output buffer is further reduced, so that the internal circuit of the semiconductor device may be more easily damaged due to static electricity. In other words, if the thickness of the gate insulating layer of the transistor is reduced, destructive voltage for the gate insulating layer is lowered. Thus, if a conventional electrostatic protection device is used, the gate insulating layer of the transistor may be destructed even if relatively lower static electricity is applied thereto. In order to solve the problem, a method for employing a transistor for the electrostatic protection device has been suggested.
FIG. 1 is a circuit diagram illustrating the conventional electrostatic protection device of a semiconductor device.
The conventional electrostatic protection device includes a delivery module 11, a control module 12, a driver 13, and a discharge module 14. The delivery module 11 induces static electricity to a line 17 of external voltage (Vcc) instead of an internal circuit in the semiconductor device through an input/output port 15. Such static electricity induced to the line 17 of the external voltage is delivered to the control module 12, the driver 13, and the discharge module 14 connected to the line 17. The control module 12 includes a resistor R1 and a capacitor C1 serially connected between the line 17 of the external voltage (Vcc) and a line 18 of the grounding voltage (Vss). The driver 13 has a CMOS type buffer including a PMOS transistor P1 and an NMOS transistor N1 serially connected between the line 17 of the external voltage (Vcc) and the line 18 of the grounding voltage (Vss). The discharge module 14 includes an NMOS transistor N2 connected between the line 17 of the external voltage VCC and the line 18 of the grounding voltage Vss.
In the electrostatic protection device for the semiconductor device, the static electricity induced to the line 17 of the external voltage (Vcc) through the delivery module 11 generates a voltage drop by means of the control module 12, and the PMOS transistor P1 of the driver 13 is turned on due to the voltage drop. As a result, the NMOS transistor N2 of the discharge module 14 is turned on, so that the line 17 of the external voltage Vcc is connected to the line 18 of the grounding voltage Vss. Accordingly, static electricity induced to the line 17 of the external voltage Vcc is discharged through the line 18 of the grounding voltage Vss. In other words, static electricity introduced into the input/output port 15 is induced to the line 17 of the external voltage Vcc and then discharged through the line 18 of the grounding voltage Vss. Accordingly, the electrostatic protection device protects the internal circuit 16 of the semiconductor device from static electricity introduced through the input/output port 15.
As described above, the conventional electrostatic protection device for a semiconductor device operates earlier than a time point of a junction breakdown. However, the conventional electrostatic protection device has a fast operation speed because a voltage drop of the control module 12 swiftly occurs. However, since the voltage drop is generated only during a rising interval of the static electricity, the operation duration of the electrostatic protection device is short. In other words, since the voltage drop of the control module 12 occurs only during a rising interval of static electricity, the driver 13 operates only during the rising interval so as to discharge static electricity. As a result, since the internal circuit of the semiconductor device is not protected from static electricity during intervals except for the rising interval of the static electricity (e.g., a peak interval of a static electricity and a falling interval of a static electricity), the internal circuit may be damaged during the intervals.
In order to solve this problem, as shown in FIG. 2, U.S. Pat. No. 5,946,177 suggests that a electrostatic protection device for a semiconductor device additionally includes a delay module 29 in order to reduce the attenuation time of voltage applied to the discharge module 24 by the driver 23.
In other words, an electrostatic protection device for a semiconductor device shown in FIG. 2 includes a delivery module 21, a control module 22, a driver 23, a discharge module 24, and a delay module 29. Static electricity introduced through an input/output port 25 is induced to the line 27 of an external voltage Vcc through the delivery module 21, and then the control module 22 performs voltage drop for the static electricity so as to operate the driver 23. Accordingly, the discharge module 24 delivers static electricity induced to the line 27 of the external voltage Vcc to a line 28 of a grounding voltage Vss, thereby discharging the static electricity. At this time, the delay module 29 delays an attenuation time of voltage applied in order to operate the discharge module 24 through the operation of a resistor element R3 and a capacitor C3. Accordingly, the electrostatic protection device shown in FIG. 2 may further increase an operation duration time as compared with the electrostatic protection device shown in FIG. 1. However, a constant value of an RC approximating to a time, in which a static electricity signal is continuously maintained, is required in order to delay voltage attenuation due to the resistor R3 and the capacitor C3 of the delay module 29. In other words, since the driver 23 does not operate after a rising duration, voltage used for operating the discharge module 24 is attenuated according to a time as charges filled in the capacitor C3 are slowly discharged through the resistor R3. Voltage according to a time is expressed as an equation,
      v    ⁡          (      t      )        =                  v              ⁢                  ⅇ                  -                      t            RC                              .      
Herein, the t, the R, the C, and |ν| denote time, resistance of a resistor, capacitance of a capacitor, and the peak value of the voltage ν(t).
Accordingly, voltage required for operating the driver 23 due to voltage drop through the control module 22 is determined by a constant value of RC obtained based on the resistor R2 and the capacitor C2, and voltage required for operating the discharge module 24 by the driver 23 is determined by a constant value of RC obtained based on the resistor R3 and the capacitor C3 of the delay module. The control module 22 must have an RC value smaller than or equal to 10 ns in order to allow the electrostatic protection device to swiftly respond to static electricity and operate, and the delay module 29 must have an RC value larger than or equal to 100 ns, which is the time interval of the electrostatic discharge pulse width.
In other words, the resistor R3 and the capacitor C3 of the delay module 29 must have values larger than or equal to 10 times values of the resistor R2 and the capacitor C2 of the control module 22. As a result, the size of the semiconductor device may increase.